153. Superconducting Cold-Electron Bolometer with JFET Readout for OLIMPO Balloon Telescope

Other conference contribution, 2005

The OLIMPO experiment is a large balloon-borne telescope, aimed at measuring the Sunyaev-Zeldovich effect in many clusters of Galaxies. OLIMPO will carry out its surveys in four frequency bands centered at 140, 220, 410 and 540 GHz. The detector system is made of four bolometer arrays. In order to achieve low dispersion in the characteristics of the detectors, a fully photo-lithographic process producing sensors on silicon nitride islands or plane Si substrate should be developed. Filters and antennas can be integrated on the detectors wafer by means of micro-strip technology. Attractive variant is to use Capacitively Coupled Cold-Electron Bolometers (CEB) with JFET readout. The JFET readout has been developed already for the BOOMERanG and Planck-HFI. The problem is to match relatively low-ohmic dynamic resistance of CEB ( 1kOhm) and high noise equivalent resistance of JFET ( 1 MOhm). The goal is to achieve level of noise-equivalen power (NEP) of CEB less than photon noise. Analysis of a single CEB with JFET readout has not given positive results in both current-biased and voltage-biased modes. Current fluctuations of JFET and feedback resistor are rather low. The main reason of fail is strong influence of voltage noise. The voltage is divided by small dynamic resistance of the junctions in cooling region (voltage-biased mode) and gives strong current noise. Any attempts to increase dynamic resistance moving to smaller voltages led to strong decrease of cooling and degradation of responsivity. To achieve noise matching with JFET, a Cold-Electron Bolometer with a weak Superconducting Absorber (SCEB) has been proposed. In this case we can operate in voltage-biased mode with voltage in the range between (Delta 1-Delta 2) and (Delta1 + Delta 2). In this region the IV of SIS´ junctions is rather flat with considerably increased dynamic resistance up to the level of Rj=1000*Rn (typical level of leakage). Electron cooling will be still very effective for incoming power. Simulations show that we can achieve photon noise level for structure with Ti absorber and Al/Ti tunnel junctions (Al antenna electrode) for all frequency ranges with estimated power load.